Many film materials, unlike paper, have no inherent capacity to absorb inks that are commonly used in printing processes. Paper however, is not a particularly durable substrate and may be damaged by handling, environmental exposure and water.
The capture of the image-forming ink on polymeric substrates presents a technical challenge because plastic film is substantially impervious to liquids. Hydrophilic coatings, applied to film materials, are known to provide receptor layers for inkjet images. Receptor layers of this type may be porous for absorbing ink droplets via capillary action. Such coatings are described, for example, in U.S. Pat. No. 5,264,275. An alternative type of absorbent inkjet receptive coating comprises polymers that swell while absorbing image forming ink droplets. Such coatings include those described in U.S. Pat. Nos. 3,889,270, 4,503,111, 4,564,560, 4,555,437, 4,379,804, 5,134,198 and 5,342,688. Hydrophilic inkjet-receptive coatings may also include multilayer coatings as described in U.S. Pat. No. 4,379,804.
For many applications however, polymeric films do not provide the same texture and handling characteristics of paper substrates. Polymeric security documents offer several benefits over their paper counterparts. In particular, polymeric banknotes can offer greatly increased durability and resistance to counterfeiting through the incorporation of security features. A requirement for polymeric banknotes is that certain physical properties are similar to the more commonly used paper banknotes. Those properties relate to tactile feel, strength, tear resistance, handling, folding, and crumple resistance.
U.S. Pat. No. 4,536,016 teaches the use of a laminate for banknotes having biaxially oriented polymeric film and a non-printed window for the incorporation of a security feature. However, U.S. Pat. Nos. 5,698,333 and 5,935,696 discuss the shortcomings of banknotes based on the '016 teachings and offers a substrate construction primarily based on a polyolefin laminate which offers improved physical properties. U.S. Pat. Nos. 5,393,099 and 5,449,200 offer yet another alternative to '016, in which a banknote is described that includes outer layers of paper laminated to a polymeric core as a way to include paper-like properties.
Polymeric banknotes offer unique opportunities to incorporate security features that are designed to discourage counterfeiting. Many patents relating to banknotes, including those cited above, mention the possibility of a transparent window somewhere on the banknote, which offers a quick visual check for authenticity and is difficult to reproduce with copying techniques. In most cases, the security feature must be added as a separate component with an additional process step.
U.S. Pat. No. 5,234,729 teaches polymeric laminates having a large number of layers and exhibiting optically unique properties. The '729 patent even suggests that the subject of that patent could be formed into plastic currency but fails to address the physical properties required for that application. See additional references U.S. Pat. Nos. 4,162,343, 4,937,134, and 5,089,318. U.S. Pat. No. 6,045,894 teaches multilayered optical films with unique optical properties that can be used as security features on certain documents of value but also fails to teach the necessary embodiments for such a film to be useful as a banknote, particularly having those physical properties required of a banknote.